1. Technical Field of the Invention
The present invention relates generally to switches. More particularly, it relates to the design and fabrication of microfabricated electromechanical switches having a single pole double throw configuration.
2. Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. PIN diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value of under 20 dB. This allows a signal to xe2x80x98bleedxe2x80x99 across the switch even when the switch is open. PIN diodes and transistors have a limited frequency response and typically only respond to frequencies under 20 GHz. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089 issued Jun. 9, 1992 to Larson discloses a microwave micro-electro-mechanical systems (MEMS) switch. The Larson MEMS switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature. This switch also provides only a Single Pole, Single Throw (SPST) function, that is, the switch is either open or closed.
U.S. Pat. No. 6.046,659 of Loo et al. discloses methods for the design and fabrication of SPST MEMS switches. Each MEMS switch has a multiple-layer armature with a suspended biasing electrode and a conducting transmission line affixed to the structural layer of the armature. A conducting dimple is connected to the conducting line to provide a reliable region of contact for the switch. The switch is fabricated using silicon nitride as the armature structural layer and silicon dioxide as a sacrificial layer supporting the armature during fabrication. Hydrofluoric acid is used to remove the silicon dioxide layer with post-processing in a critical point dryer to increase yield.
A MEMS switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB) over a large bandwidth when compared to semiconductor transistors and pin diodes. These characteristics give the MEMS switch the potential to not only replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits, but to create a whole new class of high performance and compact microwave switch circuits.
A common feature of the MEMS switches described above is that they all disclose a single pole, single throw (SPST) configuration, that is, they can only switch an RF signal on or off. However, RF signals often must be switched between two destinations, such as when switching an RF signal between a first antenna array and a second antenna array. Switches that support this configuration are classified as single pole, double throw (SPDT) switches.
SPDT switches known in the art are either solid-state devices or mechanical relays. Solid-state SPDT RF switches, such as PIN diodes and FETs, suffer from the limited frequency response, insertion loss, and isolation problems described above. Isolation between the two output ports of the SPDT switch is of particular concern, since coupling of the signal from one output port to the other output port limits the effectiveness of the switch as a dual output port device. Mechanical relays are also available in SPDT configurations, but they are generally quite large, compared to other RF components, and consume significant amounts of power.
Therefore, there is a need in the art for a SPDT switch that provides low insertion loss and high isolation at its output ports. There is a further need to provide such a switch with a size near to that of other RF components and consumes little power.
The present invention relates to a method of design and fabrication of a micro-electro-mechanical single pole double throw (SPDT) switch. The switch is preferably designed with a pair of bi-layer or tri-layer armatures which give the switch superior mechanical qualities. The switch is arranged such that one armature of the pair of armatures is normally closed while the other armature is normally open due to the application of an electrostatic potential which operates on one of the two armatures. In addition, the switch preferably has conducting dimples with defined contact areas to provide improved contact characteristics.
One embodiment of the invention is a micro-electro-mechanical switch comprising an input line, two output lines, and a pair of armatures. The input line and the output lines are located on top of a substrate. The armatures are each made of at least one structural layer, a conducting transmission line on top of, below, or between the structural layers, and a suspended armature bias electrode similarly placed of each armature. One end of the structural layer is connected to the substrate, and a substrate bias electrode is located on top of the substrate below the suspended armature bias electrode on the armatures.
The input line is coupled to a pair of input contacts, each contact of the pair of contacts being associated with one of the armatures of the pair of armatures. The output lines are each coupled to an output contact, each output contact being associated with one of the armatures of the pair of armatures. A first end of the conducting transmission line in each armature rests above each of the input contacts and a second end rests above each of the output contacts when the switch is in an open position. Each conducting transmission line also contains a conducting dimple at both the first end and the second end such that the distance between the conducting dimple and the input and output contacts is less than the distance between the conducting transmission line and the input and output contacts so that the conducting dimples contact the input and output contacts when the switch is in the closed position. The structural layer may be formed below, above, or both above and below the conducting transmission line. The input line, output lines, input contacts, output contacts, armature bias pad, substrate bias pad, and substrate bias electrode are comprised of a stack of films referred to as the first metal layer which is preferably comprised of a 1500 angstrom film of gold on top of a 100 angstrom film of nickel on top of a 900 angstrom film of gold germanium. The armature bias electrodes, conducting transmission lines, and contact dimples are made of a film stack referred to as the second metal layer, which is preferably comprised of a 1000 angstrom film of deposited or evaporated gold on top of a 200 angstrom layer of titanium. The first and second metal layers have different compositions since the first layer is deposited on the substrate while the second layer is deposited on a dielectric, such as silicon nitride.
The present invention may also be embodied in a process for making a micro-electro-mechanical switch. The process comprises a first step of depositing a first metal layer onto a substrate to form an input line, a pair of input contacts, a pair of output lines, a pair of output contacts, substrate bias electrodes, substrate bias pads, and armature bias pads. A support layer, also known as a sacrificial layer, is deposited on top of the first metal layer and the substrate, and a beam structural layer is deposited on top of the sacrificial layer. The beam structural layer forms the armature pair with one end of each armature affixed to the substrate opposite its corresponding input contact. The process further comprises the steps of removing a portion of the structural layer and a portion of the support layer to create a dimple mold. Conducting dimples are formed in the dimple mold when the conducting transmission line and suspended armature bias electrodes are fabricated by depositing a second metal layer, such that the suspended armature bias electrode is electrically connected to the armature bias pad. A second structural layer may or may not be deposited on top of the second metal layer for stress matching and thermal stability of the switch. Finally, the sacrificial layer is removed from beneath the armatures to release the armatures and allow the switch to open and close.
The materials and fabrication techniques used for the process comprise standard integrated circuit manufacturing materials and techniques. The sacrificial layer is made of silicon dioxide and is removed by wet etching the silicon dioxide with HF and with post processing in a critical point dryer. The beam structural layer is comprised of silicon nitride. As discussed above, the first metal layer is preferably comprised of a film of gold on top of a film of nickel on top of a film of gold germanium. The second metal layer is preferably comprised of a film of gold on top of film of titanium. A second beam structural layer may be deposited on top of the conducting line such that the conducting line is encased between the first structural layer and the second structural layer. In alternative embodiments of the present invention, the second metal layer is deposited underneath, in between, or on top of the structural layers. If the second metal layer is underneath the structural layers, then a dielectric or insulator is deposited on top of the substrate bias electrodes to prevent electrical shorting to the armature bias electrodes when the switch is in the closed position